crosutils/make_factory_package.sh: Support relative path
crosutils/make_factory_package.sh: Support relative path

Since there's a pushd call when building images, we have to first retrieve
the complete (full/absolute) path of target disk image.

BUG=chromium-os:10706
TEST=make_factory_package.sh \
     --factory ../build/images/x86-mario/latest/factory_image.bin \
     --release ../build/images/x86-mario/latest/chromiumos_image.bin \
     --diskimg blah_relative.bin

Change-Id: I02e2edb0f76b724337a82dd400297877681bc338

Review URL: http://codereview.chromium.org/6244003

sigma/coreos-scripts,fivethreeo/scripts,BugRoger/scripts,bcwaldon/coreos-scripts,glevand/coreos--scripts,zhang0137/scripts,vmware/coreos-scripts,zhang0137/scripts,coreos/scripts,mjg59/scripts,endocode/scripts,gtank/scripts,crawford/scripts,sigma/coreos-scripts,gtank/scripts,nicescale/scripts,gtank/scripts,smilart/scripts,coreos/scripts,mischief/scripts,andrejro/coreos--scripts,endocode/scripts,smilart/scripts,glevand/coreos--scripts,trnubo/scripts,mischief/scripts,marineam/coreos-scripts,nicescale/scripts,BugRoger/scripts,zhang0137/scripts,andrejro/coreos--scripts,BugRoger/scripts,bcwaldon/coreos-scripts,bcwaldon/coreos-scripts,trnubo/scripts,smilart/scripts,nicescale/scripts,andrejro/coreos--scripts,fivethreeo/scripts,trnubo/scripts,crawford/scripts,mjg59/scripts,marineam/coreos-scripts,marineam/coreos-scripts,fivethreeo/scripts,vmware/coreos-scripts,mjg59/scripts,sigma/coreos-scripts,mischief/scripts,vmware/coreos-scripts,crawford/scripts,endocode/scripts